This invention relates generally to sensors which are used to detect electrically conductive particulate matter, such as soot, and more particularly to a method and system for diagnosing potential failure modes in such sensors.
Incomplete combustion of certain heavy hydrocarbon compounds, such as heavy oils, diesel fuel, and the like may lead to particulate formation (e.g., soot). In the operation of internal combustion engines, excessive particulate formation can lead to “smoking” of the engine, which causes air pollution even though the carbon monoxide, hydrocarbons, and other pollutant components of the gaseous state exhaust emissions may be relatively low. Emission regulations require many engines to limit the levels of particulate emissions, and various control technologies such as diesel particulate filters (DPF) have been employed for this purpose.
In order to monitor the emissions of particulate matter (PM) in the exhaust stream of certain types of internal combustion engines, e.g., to assess the effectiveness of DPF's, it is common practice to provide a particulate sensor system for detecting the level of particulate concentration emitted from an exhaust gas. Various particulate sensors have been proposed, including those shown in U.S. Pat. No. 4,656,832 issued to Yukihisa et al., U.S. Pat. No. 6,634,210 issued to Bosch et al., U.S. Pat. Publ. No. 2008/0283398 A1, U.S. Pat. Publ. No. 2008/0282769 A1, U.S. Pat. Publ. No. 2010/0147052 A1, and U.S. Pat. No. 7,954,230 issued to Nelson, the disclosures of each of which are hereby incorporated by reference in their entirety.
Particulate sensors (also referred to herein as PM sensors or soot sensors) such as those described above generally have a pair of spaced apart sensing electrodes disposed on a substrate. The sensing electrodes are coupled to a measurement circuit by way of electrically conductive leads. The operating principle of the particulate sensor is based on the conductivity of the particulates (e.g., soot) deposited between (or over) the sensing electrodes. The electrical resistance between the sensing electrodes is relatively high when the sensor is clean but such resistance decreases as soot particulates accumulate. These sensors also have a heater that can be selectively activated to burn off the soot particulates to “reset” the sensor to a known, base “clean” state.
Government regulations require that the PM sensor have self diagnostics (i.e. On Board Diagnostics or OBD) capability to verify that it is functioning properly. Some of these diagnostics (such as sensor over temperature) require that the temperature of the sensor is known.
Accordingly, there is a need for particulate sensor diagnostics that can determine the temperature of the sensor using an existing sensor implementation, without adding an additional dedicated temperature sensor.